In the manufacture of semiconductor products and the manufacture of flat panel displays (FPDs), there is generally used a liquid, process in which a process liquid is supplied to semiconductor wafers or glass substrates as substrates to be processed. As such a process, there is a cleaning process for removing particles and contaminations adhering to substrates, for example.
As a liquid processing apparatus configured to perform such a liquid process, there is known an apparatus including a plurality of processing units of a single-wafer type, and a transfer apparatus that loads and unloads substrates to and from the processing units. Each of the processing units is configured to hold a substrate such as a semiconductor wafer on a spin chuck, and to supply a process liquid to a front surface of the substrate or a front surface and a rear surface of the substrate, while rotating the substrate so as to process the substrate. In addition, in order to achieve a higher throughput while restraining increase of a footprint of the apparatus, a layout in which these processing units are stacked at multiple levels is generally used (see, JP2005-93769A, for example).
In such a liquid processing apparatus, process-liquid supply units for supplying process liquids (chemical liquids) are disposed for respective process liquids. These process liquids are individually circulated and supplied.
JP2008-34490A describes a space-saving technique in a liquid processing apparatus including a plurality of processing units.
In addition, JP8-102457A describes a liquid replacing method in a liquid processing apparatus of a batch type which simultaneously processes a plurality of objects to be processed with the use of a liquid.
A circulation line of a process liquid (chemical liquid) has a tank for storing the process liquid. The circulation line is formed so as to extend from the tank and to return to the tank. As the liquid process progresses, not all the process liquid is returned to the tank, but a part of the process liquid is discarded. Thus, the amount of the process liquid in the tank gradually decreases. When the amount of the process liquid in the tank decreases to a certain degree, it is necessary to replenish the tank with a new process liquid.
As shown in FIG. 5, a conventional tank is provided with a liquid level sensor 101 for starting replenishment, and a liquid level sensor 102 for terminating replenishment. When the liquid level sensor for starting replenishment detects that the liquid level falls below a liquid level for starting replenishment, a replenishment of a process liquid is started. On the other hand, when the liquid level sensor for terminating replenishment detects that the liquid level exceeds a liquid level for terminating replenishment, the replenishment of the process liquid is terminated.
After the process liquid in the tank has been used for processing the predetermined number of substrates, the process liquid is replaced with a new process liquid. A conventional process-liquid replacing method is not particularly related with the aforementioned process-liquid replenishing method.